The present invention relates to a circuit board having semiconductors mounted thereon and to a CPU unit composed of a plurality of circuit boards and, in particular, to a technique of detecting false operations and failures in an operating status of the circuit board.
For high functions of recent electronic appliances, a plurality of LSIs/CPUs (microcomputers) have been packaged on a circuit board with a high density. However, problems of false operations and failures arise in the circuit board due to low voltage and high speed of electronic parts such as semiconductor integrated circuits mounted thereon.
For the high density, in particular, by an appearance of a device having a connection portion on a lower surface of a package substrate represented by a BGA (Ball Grid Array) and the like, it has been difficult to detect the failures visually and by making a probe contact with the device from the outside.
Therefore, as described in, for example, Japanese Patent Laid-open Publication No. 2002-296315 (Patent Document 1), the failures are conventionally detected by providing, outside the circuit board, a command generation function for inspecting functions of the circuit board and an inspection function for inspecting an output from the circuit board.
However, when the circuit board is mounted inside an apparatus, since the external inspection function cannot be connected, the detecting is limited to initial defects in manufacture before the circuit board is mounted on the apparatus and to the failures in a time of the inspection. Therefore, it is difficult to detect the false operations in the operating status.
Further, as a method of providing the inspection function on the circuit board, there is a digital boundary scan test according to IEEE Standard 1149.1. In this method, a digital boundary scan cell and a TAP (Test Access Port) that receives a command and controls the boundary scan cell are built in the semiconductor integrated circuit mounted on the board, thereby diagnosing connection of the semiconductor integrated circuit by sending a few kinds of signals such as TDI (Test Data In), TDO (Test Data Out), TMS (Test Mode Select), and TCK (Test Clock).